Field of the Invention
Applicant's invention relates to devices and methods for reducing inhomogeneities in the magnetic field in a magnetic resonance imaging (“MRI”) system. More particularly, it relates to using automated devices and methods to set and maintain a pre-determined current for each of one or more shim coils.
Background Information
MRI scanners are used to image and help diagnose patients with various health problems. It allows for the imaging of internal organs and structures inside a living body. An MRI device uses a powerful magnetic field. In order to create the magnetic field, MRI devices use a strong magnet. When a person is inside the magnetic field of the scanner, the magnetic moments of some of the molecules in the person's body become aligned, in an equilibrium state, with the direction of the field. Pulses of radio wave energy are applied to produce an additional electromagnetic field. Photons in this field are disturbed from their equilibrium state. When the field is removed, the protons in different tissues return to their equilibrium state, but at different rates. The difference in the return can be detected, recorded, and used to create an image.
In magnetic resonance (“MR”) imaging, the homogeneity of the magnetic field is critically important to producing high quality images. Homogeneity refers to the uniformity of the magnetic field over a specified volume. A homogenous magnetic field has little or no difference between the maximum and minimum field strengths across a specified volume. Left uncorrected, inhomogeneities in the magnetic field cause distortions in the images, which in turn cause images of patient anatomies to be less clear.
Unfortunately, the magnet's magnetic field tends to be very inhomogeneous. In order to improve the homogeneity of the magnetic field in the imaging area, MRI devices are adjusted. The process of adjusting the magnetic field is referred to as “shimming,” and it can be accomplished by using either passive or active means.
When an MRI device is first installed, ferrous metal plates or wedges—shims—may be placed at various positions around the magnets to bias, or shift the curvature of, the magnetic field. This process is passive shimming. Typically, in passive shimming, the ferrous metal is in the form of small iron plates which are held in removable trays. Before shimming, the user measures and plots the unadjusted magnetic field and then places shims in calculated positions to influence and correct the inhomogeneities in the magnetic field. Then, the user may repeat the process of measuring the adjusted field and placing shims until the desired field homogeneity is achieved.
Passive shims can compensate for factors such as imperfections in the magnet or the presence of other ferrous metal objects outside the MRI device. After the patient is placed inside the bore, the user can measure the magnetic field and then calculate the number and position of passive shims needed to compensate for the changes in the magnetic field caused by the individual patient.
Many MRI system manufacturers also provide for active shimming to reduce the inhomogeneities to a level sufficient to provide high quality images. Active shimming uses smaller magnetic coils to produce magnetic fields that interact with the primary field. In active shimming these smaller shim coils are energized with relatively low currents that create auxiliary magnetic fields which compensate for inhomogeneities in the main magnetic field. For example, one MRI system has between twelve and eighteen shim channels used to calibrate the magnetic imaging field.
As used herein, a shim channel consists of a shim coil, powered by a highly stable power supply, with the associated power supply output controls. With the help of a shim power supply, users are able to adjust the magnetic field. The shim power supply (“SPS”) is a portable piece of test and calibration equipment that often contains six individual direct current (“DC”) power supplies and a separate switch heater power supply. An SPS typically enables the user to monitor and adjust six shim channels during the calibration process. Users often adjust a total of eighteen shim channels, six channels for each of three main coils in the MRI.
The SPS front panel design for one model includes a fine adjustment knob and a coarse adjustment knob to vary the DC current supplied to the shim coil for each channel. To achieve the desired current in a particular shim coil, the user must first manually turn the coarse adjustment knob to set a current level near the desired level, and then manually turn the fine adjustment knob to set the desired output for the channel. The user must then repeat this tedious process for each channel. However, during this calibration process, the power supplies may “drift” away from the desired current, and the user must retune each of the drifting channels multiple times in order to get the most accurate calibration.
By combining passive and active shimming, the user can improve the homogeneity of the magnetic field in the imaging area. However, these processes involve multiple steps and may take several hours to correct the magnetic field to within OEM specifications and may not completely eliminate the inhomogeneities in the MRI magnetic field.